Chromatin in SSCs Résumé de rapport

Final Report Summary - CHROMATIN IN SSCS (4D analysis of chromatin dynamics during the early stages of spermatogenesis: A journey to the stem of male infertility.)

One in six couples is infertile and in about half of these cases the male partner suffers from impaired spermatogenesis, with spermatogenic arrest causing azoospermia being the most severe condition. There are some known causes for spermatogenic arrest, including previous chemo- or radiotherapy, structural and numerical chromosomal abnormalities and Y-chromosome deletions. However, the etiology of spermatogenic arrest remains unknown in most cases. Moreover, no treatment options for infertile males are currently available.In my Marie Curie (MC) project I aimed to investigate how spermatogenetic arrest is initiated and how chromatin dynamics determine development, genomic integrity maintenance and survival of male reproductive cells. For my research I was able to implement my expertise in spermatogenesis, chromatin biology and imaging in the available cell culture models of rodent and human reproductive cells and patient material of the host institution. The studies in this report have been carried out at the host institution under my supervision by the team I was able to build up during the project. During the first phase of the MC we characterized the presence of various Structural Maintenance of Chromosomes (SMC) proteins during spermatogenesis and decided to focus our efforts on Smc6; a protein that has been shown to be involved in genomic damage responses in yeast and drosophila model systems and appeared to be highly expressed in the testis. We then analyzed the role of Smc6 during rodent spermatogenesis and found it to be involved in pericentromeric heterochromatin function during spermatogonial differentiation and meiosis, the germ cell specific cell divisions generating haploid cells. Because pericentromeric heterochromatin embeds highly repetitive sequences, meiotic recombination should be suppressed in these regions to avoid chromosomal aberrations that could lead to apoptosis, meiotic arrest or aneuploidies. In our first publication (Verver et al., 2013) we describe how Smc6 can fulfil this role during mammalian meiosis. We subsequently investigated whether Smc6 would have a similar function during human spermatogenesis and did two surprising findings (Verver et al., 2014). First, we characterized a subpopulation of differentiating spermatogonia that was until now not distinguishable in the human. Secondly, we found that human pericentromeric heterochromatin is not protected against aberrant meiotic recombination by Smc6. The latter we attribute to the more central localization and different fragmented nature of the repetitive sequences of the human pericentromeric regions. In parallel, using cultured spermatogonia, we are currently analyzing the biochemical regulation of Smc6 in relation to genomic damage and chromosomal organization. In parallel a PhD student has started to study the influence of genomic stability during early embryo implantation. Moreover, a fourth student will start this year on a project about chromosome stability and aneuploidies in oocytes.These studies fit very well with recently granted projects in my team on meiotic arrest mechanisms in the human, ovarian aging, early embryo implantation and CRISPR-mediated gene modifications of mouse germ cells. The MC has contributed significantly to the expansion of my team by giving me financial relieve allowing us to visit conferences and other laboratories, purchase required equipment and reagents and the time for teaching and other outreach activities. In turn this has led to connections with undergraduate students for internships in my group and novel international collaborations that have led to a review article (Verver et al 2015) and will propel our research in a forward direction.